Electrical connectors are essential to any electrical or electronic system. Designers of electronic component ranging from portable consumer electronics to massive computer platforms have constantly strived to find the highest performing, lowest cost connectors. The applications may vary, from plugging modules into a back plane to plugging a set of headphones into a portable media player, but the goal is the same, to provide the best connection at the lowest cost.
Recent trends in the industry have tended to reduce the number of interconnect pins on the integrated circuits employed in systems. One method of reducing the number of pins is to replace parallel signal interfaces with high speed serial channels that transfer data with equal or greater bandwidth using fewer signals pins. However, to achieve the same bandwidth as a parallel interface, the signals on a serial interface need to run at faster speed. For example, when a 16-bit parallel interface, consisting of 16 individual data channels, is replaced by a single serial channel, the serial channel will have to operate at 16× the speed of one of the parallel channels.
Consequently, to achieve the speeds required, the serial lines may have to employ differential signaling techniques. Differential signaling techniques allow signals to run at significantly higher speeds than do single-ended techniques. This results in an overall reduction of the number of connections required to provide the bandwidth necessary for the interface.
The adoption of high-speed serial interconnects has resulted in significant increases in interface speeds reaching in excess of 10 Gigabits per second (Gb/s). The increase in data rates has, however, introduced a new set of problems many of which relate to signal integrity.
One of the most important factors in the design of a system interconnect is what is known as signal integrity. Signal integrity refers to the quality of the signal at the receiving end of the network and, therefore, it determines the maximum speed at which the channel can transfer data. Example factors that affect the signal integrity of an interconnect are: component variation, material variations, power distribution, signal crosstalk, PCB layout, PCB construction and impedance discontinuities. Most of these factors can be addressed with good design and manufacturing techniques for given components or materials. However, the ubiquity of signal connectors has made impedance discontinuities an issue that has received considerable industry attention.
There are 3 primary places in an interconnect where impedance discontinuities arise. These are: (i) the module to connector connection on the device; (ii) connector to receptacle connector contact; and (iii) the module to connector connection on the host side.
A common impedance discontinuity arises when the cross-section of the conducting element changes. As those of skill will appreciate, when a conducting element presents two cross sections, the first cross-section of the conducting element has a first characteristic impedance while the second cross-section of the conducting element has a second and typically different characteristic impedance.
This creates two physically dissimilar transmission lines and causes distortion in the fields and a small mismatch, even for lines of like characteristic impedance (Zo).
Another impedance discontinuity arises where connectors do not have a “through” characteristic impedance that matches the transmission line. Not only may there be geometric discontinuities, the length of the different sections with different characteristic impedances may create significant signal reflection which adversely affects on signal integrity. As those of skill will recognize, a section of mismatched line in an otherwise matched system changes the impedance looking into the mismatched section.
From an impedance perspective, connectors are complex. Their individual components can each contribute to make uncertain what might be thought an easily determined impedance value. The leading edge of a signal propagating down the transmission line model of a connector acts like a wave. When a wave hits an impedance discontinuity, a portion of the wave is going to continue propagating while a portion is reflected back toward the source. The percentage of the wave reflected is related to the difference in characteristic impedance of two (2) segments. The greater the discontinuity, the greater the reflection. In short, impedance deviations cause signal reflections and impair transmission characteristics. Consequently, what is needed is a new design compliant with existing standards but which presents a cleaner impedance path to signal flow.